<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="en" dir="ltr" class="client-nojs">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Non-static data members - cppreference.com</title>
<meta charset="UTF-8">
<meta name="generator" content="MediaWiki 1.21.2">


<link rel="shortcut icon" href="../../../favicon.ico">



<link rel="stylesheet" href="../../../common/ext.css">
<meta name="ResourceLoaderDynamicStyles" content="">
<link rel="stylesheet" href="../../../common/site_modules.css">
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-css:7:472787eddcf4605d11de8c7ef047234f */</style>

<script src="../../../common/startup_scripts.js"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/language/data_members","wgTitle":"cpp/language/data members","wgCurRevisionId":80216,"wgArticleId":9910,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":[],"wgBreakFrames":false,"wgPageContentLanguage":"en","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"dmy","wgMonthNames":["","January","February","March","April","May","June","July","August","September","October","November","December"],"wgMonthNamesShort":["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"wgRelevantPageName":"cpp/language/data_members","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"en","language":"en","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-js:7:ca03345b1e2c4d90a25d968753a73b92 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
	line-height: normal; border: 0px none white;
}
/**
 * GeSHi Dynamically Generated Stylesheet
 * --------------------------------------
 * Dynamically generated stylesheet for cpp
 * CSS class: source-cpp, CSS id: 
 * GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
 * (http://qbnz.com/highlighter/ and http://geshi.org/)
 * --------------------------------------
 */
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp  {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }

/*]]>*/
</style>
<!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]-->
</head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_data_members skin-cppreference2 action-view cpp-navbar">
        <!-- header -->
        
        <!-- /header -->
        <!-- content -->
        <div id="cpp-content-base">
            <div id="content">
                <a id="top"></a>
                <div id="mw-js-message" style="display:none;"></div>
                                <!-- firstHeading -->
                <h1 id="firstHeading" class="firstHeading">Non-static data members</h1>
                <!-- /firstHeading -->
                <!-- bodyContent -->
                <div id="bodyContent">
                                        <!-- tagline -->
                    <div id="siteSub">From cppreference.com</div>
                    <!-- /tagline -->
                                        <!-- subtitle -->
                    <div id="contentSub"><span class="subpages">&lt; <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../language.1.html" title="cpp/language">language</a></span></div>
                    <!-- /subtitle -->
                                                            <!-- bodycontent -->
                    <div id="mw-content-text" lang="en" dir="ltr" class="mw-content-ltr">
<div class="t-navbar" style="">
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head">
<a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div>
<div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.1.html" title="cpp/language"> Language</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header"> Standard library headers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concept.html" title="cpp/concept"> Concepts</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility"> Utilities library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string"> Strings library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../container.html" title="cpp/container"> Containers library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm"> Algorithms library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator"> Iterators library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric"> Numerics library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io"> Input/output library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale"> Localizations library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex"> Regular expressions library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic"> Atomic operations library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread"> Thread support library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../experimental.html" title="cpp/experimental"> Technical Specifications</a> </td></tr>
</table></div>
<div></div>
</div></div>
</div>
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head"><a href="../language.1.html" title="cpp/language"> C++ language</a></div>
<div class="t-navbar-sep"> </div>
<div class="t-navbar-head">
<a href="classes.html" title="cpp/language/classes"> Classes</a><div class="t-navbar-menu"><div>
<div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> General </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> overview</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> class/struct types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="union.html" title="cpp/language/union"> union types</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Members </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">data members</strong>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static">static members</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this">the <code>this</code> pointer</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">nested classes</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">member templates</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bit_field.html" title="cpp/language/bit field">bit fields</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration">using-declarations</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_functions.html" title="cpp/language/member functions">member functions</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access">member access specifiers</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="constructor.html" title="cpp/language/initializer list">constructors and member initializer lists</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">in-class <i>brace-or-equal</i> initializer</a><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"><code>friend</code> specifier</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code> specifier</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">converting constructor</a>
</td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">default constructor</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator">copy assignment operator</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator">move assignment operator</a><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">destructor</a>
</td></tr>
<tr class="t-nv-h2"><td colspan="5"> Inheritance </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="derived_class.html" title="cpp/language/derived class">base and derived classes</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">virtual member functions</a>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override">override</a><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final">final</a><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">pure virtual functions and abstract classes</a>
</td></tr>
</table></div>
</div>
<div></div>
</div></div>
</div>
<div class="t-navbar-sep"> </div>
</div>
<p>Non-static data members are the variables that are declared in a <a href="class.html" title="cpp/language/class">member specification</a> of a class.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> S <span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span> <span class="co1">// non-static data member</span>
    <span class="kw4">int</span><span class="sy3">&amp;</span> r<span class="sy4">;</span> <span class="co1">// non-static data member of reference type</span>
    <span class="kw4">int</span> a<span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// non-static data member with initializer (C++11)</span>
    <a href="../string/basic_string.html"><span class="kw1049">std::<span class="me2">string</span></span></a> s, <span class="sy2">*</span>ps<span class="sy4">;</span> <span class="co1">// two non-static data members</span>
    <span class="kw1">struct</span> NestedS <span class="br0">{</span>
        <a href="../string/basic_string.html"><span class="kw1049">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
    <span class="br0">}</span> d5, <span class="sy2">*</span>d6<span class="sy4">;</span> <span class="co1">// two non-static data members of nested type</span>
    <span class="kw4">char</span> bit <span class="sy4">:</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// two-bit bitfield</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Any <a href="declarations.html" title="cpp/language/declarations">simple declarations</a> are allowed, except
</p>
<ul>
<li> <a href="../keyword/extern.html" title="cpp/keyword/extern">extern</a> and <a href="../keyword/register.html" title="cpp/keyword/register">register</a> storage class specifiers are not allowed
</li>
<li> <a href="../keyword/thread_local.html" title="cpp/keyword/thread local">thread_local</a> storage class specifier is not allowed (but it is allowed for <a href="static.html" title="cpp/language/static">static</a> data members)
</li>
<li> <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete types</a> are not allowed: in particular, a class <code>C</code> cannot have a non-static data member of class <code>C</code>, although it can have a non-static data member of type <code>C&amp;</code> (reference to C) or <code>C*</code> (pointer to C)
</li>
<li> a non-static data member cannot have the same name as the name of the class if at least one user-declared constructor is present.
</li>
</ul>
<p>In addition, <a href="bit_field.html" title="cpp/language/bit field">bit field declarations</a> are allowed.
</p>
<p>Non-static data members are allowed to have the same name as the name of the enclosing class unless the class has a user-declared constructor.
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="data_members.html#Layout"><span class="tocnumber">1</span> <span class="toctext">Layout</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="data_members.html#Standard_layout"><span class="tocnumber">2</span> <span class="toctext">Standard layout</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="data_members.html#Member_initialization"><span class="tocnumber">3</span> <span class="toctext">Member initialization</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="data_members.html#Usage"><span class="tocnumber">4</span> <span class="toctext">Usage</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="data_members.html#See_also"><span class="tocnumber">5</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Layout">edit</a>]</span> <span class="mw-headline" id="Layout">Layout</span>
</h3>
<p>When an object of some class <code>C</code> is created, each non-static data member of non-reference type is allocated in some part of the object representation of <code>C</code>. Whether reference members occupy any storage is implementation-defined.
</p>
<p>For non-<a href="union.html" title="cpp/language/union">union</a> class types, members with the same <a href="access.html" title="cpp/language/access">member access</a> are always allocated so that the members declared later have higher addresses within a class object. Members with different access control are allocated in unspecified order (the compiler may group them together). Alignment requirements may necessitate padding between members, or after the last member of a class.
</p>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Standard layout">edit</a>]</span> <span class="mw-headline" id="Standard_layout">Standard layout</span>
</h3>
<p>A class where all non-static data members have the same access control and certain other conditions are satisfied is known as <i>standard layout type</i> (see <a href="../concept/StandardLayoutType.html" title="cpp/concept/StandardLayoutType"><code>StandardLayoutType</code></a> for the list of requirements).
</p>
<p>Two standard-layout non-union class types may have a <i>common initial sequence</i> of non-static data members<span class="t-rev-inl t-since-cxx17"><span> and bit-fields</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>, for a sequence of one or more initial members (in order of declaration), if the members have layout-compatible types<span class="t-rev-inl t-since-cxx17"><span> and if they are bit-fields, they have the same width</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span> <span class="kw4">int</span> a<span class="sy4">;</span> <span class="kw4">char</span> b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw4">const</span> <span class="kw4">int</span> b1<span class="sy4">;</span> <span class="kw4">volatile</span> <span class="kw4">char</span> b2<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> 
<span class="co1">// A and B's common initial sequence is A.a, A.b and B.b1, B.b2</span>
<span class="kw1">struct</span> C <span class="br0">{</span> <span class="kw4">int</span> c<span class="sy4">;</span> <span class="kw4">unsigned</span> <span class="sy4">:</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="kw4">char</span> b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// A and C's common initial sequence is A.a and C.c</span>
<span class="kw1">struct</span> D <span class="br0">{</span> <span class="kw4">int</span> d<span class="sy4">;</span> <span class="kw4">char</span> b <span class="sy4">:</span> <span class="nu0">4</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// A and D's common initial sequence is A.a and D.d</span>
<span class="kw1">struct</span> E <span class="br0">{</span> <span class="kw4">unsigned</span> <span class="kw4">int</span> e<span class="sy4">;</span> <span class="kw4">char</span> b<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// A and E's common initial sequence is empty</span></pre></div></div>
<p>Two standard-layout non-union class types are called <i>layout-compatible</i> if they are the same type <span class="t-rev-inl t-since-cxx17"><span>ignoring cv-qualifiers, if any</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>, are layout-compatible <a href="enum.html" title="cpp/language/enum">enumerations</a>, or if their <i>common initial sequence</i> consists of every non-static data member<span class="t-rev-inl t-since-cxx17"><span> and bit field</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> (in the example above, <code>A</code> and <code>B</code> are layout-compatible)
</p>
<p>Two standard-layout unions are called <i>layout-compatible</i> if they have the same number of non-static data members and corresponding non-static data members (in any order) have layout-compatible types.
</p>
<p>Standard layout types have the following special properties:
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx17">
<td>
<dl><dd>
<ul><li> If a standard-layout <a href="union.html" title="cpp/language/union">union</a> holds two (or more) standard-layout structs as members, and these structs have a common initial sequence of data members, it is well-defined to examine any member of that common initial sequence regardless of which member of the union is active.
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx17">(until C++17)</span></td>
</tr>
<tr class="t-rev t-since-cxx17">
<td>
<dl><dd>
<ul><li> In a standard-layout union with an active member of non-union class type <code>T1</code>, it is permitted to read a non-static data member <code>m</code> of another union member of non-union class type <code>T2</code> provided <code>m</code> is part of the common initial sequence of <code>T1</code> and <code>T2</code> (except that reading a volatile member through non-volatile glvalue is undefined)
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td>
</tr>
</table>
<dl><dd>
<ul>
<li> A pointer to an object of standard-layout struct type can be <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast">reinterpret_cast</a> to pointer to its first non-static data member (if it has non-static data members) or otherwise its first base class subobject (if it has any), and vice versa. (padding is not allowed before the first data member). Note that <a href="reinterpret_cast.html#Type_aliasing" title="cpp/language/reinterpret cast">strict aliasing</a> rules still apply to the result of such cast.
</li>
<li> The macro <span class="t-lc"><a href="../types/offsetof.html" title="cpp/types/offsetof">offsetof</a></span> may be used to determine the offset of any member from the beginning of a standard-layout struct
</li>
</ul>
</dd></dl>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Member initialization">edit</a>]</span> <span class="mw-headline" id="Member_initialization">Member initialization</span>
</h3>
<p>Non-static data members may be initialized in one of two ways:
</p>
<div class="t-li1">
<span class="t-li">1)</span> In the <a href="constructor.html" title="cpp/language/initializer list">member initializer list</a> of the constructor.
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1049">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
    S<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span> <span class="co1">// direct-initializes n, default-initializes s</span>
    <span class="br0">{</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11">
<td>
<div class="t-li1">
<span class="t-li">2)</span> Through a brace-or-equal initializer, which is simply an <a href="initialization.html" title="cpp/language/initialization">initializer</a> included in the member declaration, which is used if the member is omitted in the member initializer list
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1049">std::<span class="me2">string</span></span></a> s<span class="br0">{</span><span class="st0">'a'</span>, <span class="st0">'b'</span>, <span class="st0">'c'</span><span class="br0">}</span><span class="sy4">;</span>
    S<span class="br0">(</span><span class="br0">)</span> <span class="co1">// copy-initializes n, list-initializes s</span>
    <span class="br0">{</span> <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<p>If a member has a brace-or-equal initializer and also appears in the member initialization list in a constructor, the brace-or-equal initializer is ignored.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="sy2">++</span>x<span class="sy4">;</span>
    S<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// uses brace-or-equal initializer</span>
    S<span class="br0">(</span><span class="kw4">int</span> arg<span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span>arg<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// uses member initializer list</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1453">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints 0</span>
    S s1<span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1453">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints 1</span>
    S s2<span class="br0">(</span><span class="nu0">7</span><span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/cout.html"><span class="kw1453">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints 1</span>
<span class="br0">}</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td>
</tr>
<tr class="t-rev t-since-cxx17">
<td>
<p>Reference members cannot be bound to temporaries in a brace-or-equal initializer
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A <span class="br0">{</span>
    A<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span>          <span class="co1">// OK</span>
    A<span class="br0">(</span><span class="kw4">int</span> v<span class="br0">)</span> <span class="sy4">:</span> v<span class="br0">(</span>v<span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>     <span class="co1">// OK</span>
    <span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> v <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>      <span class="co1">// OK</span>
<span class="br0">}</span><span class="sy4">;</span>
A a1<span class="sy4">;</span>    <span class="co1">// error: ill-formed binding of temporary to reference</span>
A a2<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK (brace-or-equal initializer ignored because v appears in a constructor)</span>
         <span class="co1">// however a2.v is a dangling reference</span></pre></div></div>
<p>Reference members cannot be initialized with brace-or-equal-initializers if it has a subexpression that would execute aggregate initialization which would use the same initializer:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> A<span class="sy4">;</span>
<span class="kw4">extern</span> A a<span class="sy4">;</span>
<span class="kw1">struct</span> A <span class="br0">{</span>
   <span class="kw4">const</span> A<span class="sy3">&amp;</span> a1 <span class="br0">{</span> A<span class="br0">{</span>a,a<span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK</span>
   <span class="kw4">const</span> A<span class="sy3">&amp;</span> a2 <span class="br0">{</span> A<span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>    <span class="co1">// error</span>
<span class="br0">}</span><span class="sy4">;</span>
A a<span class="br0">{</span>a,a<span class="br0">}</span><span class="sy4">;</span>                  <span class="co1">// OK</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td>
</tr>
</table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Usage">edit</a>]</span> <span class="mw-headline" id="Usage">Usage</span>
</h3>
<p>The name of a non-static data member or a non-static member function can only appear in the following three situations:
</p>
<div class="t-li1">
<span class="t-li">1)</span> As a part of class member access expression, in which the class either has this member or is derived from a class that has this member, including the implicit <code>this-&gt;</code> member access expressions that appear when a non-static member name is used in any of the contexts where <a href="this.html" title="cpp/language/this">this</a> is allowed (inside member function bodies, in member initializer lists, in the in-class brace-or-equal initializers)
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
   <span class="kw4">int</span> m<span class="sy4">;</span>
   <span class="kw4">int</span> n<span class="sy4">;</span>
   <span class="kw4">int</span> x <span class="sy1">=</span> m<span class="sy4">;</span>  <span class="co1">// OK: implicit this allowed in brace-or-equal initializers (C++11)</span>
   S<span class="br0">(</span><span class="kw4">int</span> init<span class="br0">)</span> <span class="sy4">:</span> m<span class="br0">(</span>init<span class="br0">)</span>,
                 n<span class="br0">(</span>m<span class="br0">)</span> <span class="co1">// OK:implicit this allowed in member initializer lists</span>
   <span class="br0">{</span>
       this<span class="sy2">-</span><span class="sy1">&gt;</span>f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// explicit member access expression</span>
       f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// implicit this allowed in member functions</span>
   <span class="br0">}</span>
   <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</div>
<div class="t-li1">
<span class="t-li">2)</span> To form a <a href="pointer.html" title="cpp/language/pointer">pointer to member</a>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
   <span class="kw4">int</span> m<span class="sy4">;</span>
   <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> S<span class="sy4">::</span><span class="sy2">*</span>p <span class="sy1">=</span> <span class="sy3">&amp;</span>S<span class="sy4">::</span><span class="me2">m</span><span class="sy4">;</span> <span class="co1">// OK, use of m to make a pointer to member</span>
<span class="kw4">void</span> <span class="br0">(</span>S<span class="sy4">::</span><span class="sy2">*</span>fp<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="sy3">&amp;</span>S<span class="sy4">::</span><span class="me2">f</span><span class="sy4">;</span> <span class="co1">// OK, use of f to make a pointer to member</span></pre></div></div>
</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11">
<td>
<div class="t-li1">
<span class="t-li">3)</span> in unevaluated operands
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
   <span class="kw4">int</span> m<span class="sy4">;</span>
   <span class="kw4">static</span> <span class="kw4">const</span> <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> sz <span class="sy1">=</span> sizeof m<span class="sy4">;</span> <span class="co1">// OK, m in unevaluated operand</span>
<span class="br0">}</span><span class="sy4">;</span>
<a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> j <span class="sy1">=</span> sizeof<span class="br0">(</span>S<span class="sy4">::</span><span class="me2">m</span> <span class="sy2">+</span> <span class="nu0">42</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK, even though there is no "this" object for m</span></pre></div></div>
</div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td>
</tr>
</table>
<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span>
</h3>
<ul>
<li>
<a href="classes.html" title="cpp/language/classes">classes</a>
</li>
<li>
<a href="static.html" title="cpp/language/static">static data members</a>
</li>
<li>
<a href="member_functions.html" title="cpp/language/member functions">member functions</a>
</li>
<li>
<span class="t-lc"><a href="../types/is_standard_layout.html" title="cpp/types/is standard layout">std::is_standard_layout</a></span>
</li>
<li>
<span class="t-lc"><a href="../types/offsetof.html" title="cpp/types/offsetof">offsetof</a></span>
</li>
</ul>

<!-- 
NewPP limit report
Preprocessor visited node count: 1358/1000000
Preprocessor generated node count: 3749/1000000
Post‐expand include size: 40719/2097152 bytes
Template argument size: 8326/2097152 bytes
Highest expansion depth: 13/40
Expensive parser function count: 0/100
-->

<!-- Saved in parser cache with key mwiki1-mwiki_en_:pcache:idhash:9910-0!*!0!!en!*!* and timestamp 20150731221308 -->
</div>                    <!-- /bodycontent -->
                                        <!-- printfooter -->
                    <div class="printfooter">
                    Retrieved from "<a href="http%3A//en.cppreference.com/mwiki/index.php">http://en.cppreference.com/mwiki/index.php?title=cpp/language/data_members&amp;oldid=80216</a>"                    </div>
                    <!-- /printfooter -->
                                                            <!-- catlinks -->
                    <div id="catlinks" class="catlinks catlinks-allhidden"></div>                    <!-- /catlinks -->
                                                            <div class="visualClear"></div>
                    <!-- debughtml -->
                                        <!-- /debughtml -->
                </div>
                <!-- /bodyContent -->
            </div>
        </div>
        <!-- /content -->
        <!-- footer -->
        
        <!-- /footer -->
        <script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../common/skin_scripts.js"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../common/site_scripts.js"></script>

<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 0.081 secs. -->
	</body>
<!-- Cached 20150731221309 -->
</html>
